Film and television image generation systems give rise to the appearance to the viewer of continuously moving visual images. Actually, the appearance of continuous motion results from visual and mental integration by the viewer of rapidly advancing sequences of still frame images.
Conventionally, in countries having a 60 Hz primary power distribution frequency for the power grid, motion picture films are generated and are projected at one frame rate, such as 24 film frames per second, while television images are generated and displayed at another television image frame rate, such as 30 television frames per second (in the standard NTSC format, the 30 frames per second are comprised of 60 interlaced fields per second, or more precisely 59.94 fields per second within the NTSC color standard signal format).
Line scan doubling techniques have been proposed to overcome some of the drawbacks resulting from the early adoption of the 262.5 line per field scan standard for television. When the number of scan lines per field is doubled, and the result is presented in progressive scan per field format, improved visual appearance, particularly with large screen display formats, is realized.
In U.S. Pat. application Ser. No. 07/262,321 filed on Oct. 25, 1988, for "Improved Film to Video Converter with Scan Line Doubling", now U.S. Pat. No. 4,876,596, the assignee of the present invention describes methods and apparatus for controlling a scan line doubler to eliminate undesirable shimmering artifacts otherwise associated with video images which have originated from three to two, pull down ratio film conversion. The disclosure of U.S. Pat. No. 4,876,596 is specifically incorporated herein by reference.
The incorporated patent application describes apparatus for selecting time compressed frames of video in a manner which overcomes the shimmering artifact. The selection of time compressed frames was controlled by a state machine which generated a recurrent sequence of selection states described in the incorporated application based upon the particular pull down ratio of film to video and based upon the instantaneous position of a frame within the sequence.
More recently, progressively scanning cameras, operating at 30 Hz per frame without interlace, are being used to generate video image sequences which lend themselves to line doubling techniques. Thirty frame-per-second (30 FPS) film has also been a source of images particularly well suited to 60 Hz television signals generation. A need has arisen for a convenient mechanism for detecting the occurrence of a 30 Hz sequential video image sequence, in order thereby to control line doubling apparatus more efficiently and effectively.
A hitherto unsolved need has remained for a controller which automatically determines the presence of video images which progress within a unique sequence which indicates film to video transfer, or progressively scanned video frames, as the case may be.